The present invention relates to a method for measuring the corrosivity of liquids towards primarily metals and to a device for carrying out the method.
When storing petroleum products there are in some cases formed various sulphur compounds because of, for instance, bacterial influence, which compounds will attack metals. These corrosive compounds will form on the surface of the metal a film of molecules consisting of atoms from the corroding metal but also consisting of sulphur, hydrogen etc. This film increases on the metal and may cause big problems. This is especially relevant for oil products, e.g. crude oils, oils for heating purposes and aviation fuels, that are in contact with water. This has been observed when oil products are stored inside rock chambers on a water bed, and this is a serious problem especially when jet fuel is being stored. In the interface between water and oil there are microorganisms, e.g. sulphate-reducing bacteria, that will form sulphur compounds, such as hydrogen sulphide, elementary sulphur and certain organic sulphur compounds, including among others mercaptans, sulphides and disulphides. Said compounds will move upwards into the oil and make it corrosive. If such corrosive oil is used as motor fuel, the motor among other things may become damaged because certain metal parts will corrode, and if the fuel is aviation fuel the corrosion constitutes a potential danger for the safety in flight.
This is why it is very important to have a possibility of determining the corrosivity in an easy but at the same time reliable way, preferably at field locations. Particularly when storing for instance jet fuel over a long period in a rock chamber, there is a great need to be able to discover at a very early stage if the fuel is becoming corrosive. The method should therefore preferably be so adapted that an analysis can be carried out at the very place, for instance at a rock chamber.
It is here not necessary to exactly know the chemical composition of the corrosive compounds. It is fully adequate to learn about for instance how rapidly the surface film, which is formed on a metal immersed in the petroleum product, will increase.
The method that is used today to measure the corrosivity of a petroleum product is an ocular inspection of a metal strip, preferably made of silver, which has been kept in the product in question. A silver strip, which is corroded by sulphur compounds in the petroleum product, becomes black, and a classification from 0 to 4 is used as a graduation of the blackening extent. Even if the test method that is used today is standardized, IP 227/73, 1976, Institute of Petroleum, Standards for petroleum and its products, Methods for analysis and testing, 35th ed., Applied Science Publishers, a subjective estimation is made of the colouration and thus of the corrosiveness. Besides, the method takes a long time to perform, relatively large sample volumes (250 ml) are required, and the classification with five steps is too coarse. Furthermore, a colouration recognizable for the eye cannot be observed until the film thickness exceeds about 100 .ANG.. Therefore it is desirable to have a more rapid and less subjective test method that is more sensitiveand more reproducible than the methods used up to now.